socket - Linux socket interface
#include <sys/socket.h>
sockfd = socket(int socket_family, int socket_type, int protocol);
This manual page describes the Linux networking socket layer user interface. The
BSD compatible sockets are the uniform interface between the user process and
the network protocol stacks in the kernel. The protocol modules are grouped
into
protocol families such as
AF_INET,
AF_IPX, and
AF_PACKET, and
socket types such as
SOCK_STREAM or
SOCK_DGRAM. See
socket(2) for more information on families and
types.
These functions are used by the user process to send or receive packets and to
do other socket operations. For more information, see their respective manual
pages.
socket(2) creates a socket,
connect(2) connects a socket to a
remote socket address, the
bind(2) function binds a socket to a local
socket address,
listen(2) tells the socket that new connections shall
be accepted, and
accept(2) is used to get a new socket with a new
incoming connection.
socketpair(2) returns two connected anonymous
sockets (implemented only for a few local families like
AF_UNIX)
send(2),
sendto(2), and
sendmsg(2) send data over a socket,
and
recv(2),
recvfrom(2),
recvmsg(2) receive data from a
socket.
poll(2) and
select(2) wait for arriving data or a
readiness to send data. In addition, the standard I/O operations like
write(2),
writev(2),
sendfile(2),
read(2), and
readv(2) can be used to read and write data.
getsockname(2) returns the local socket address and
getpeername(2)
returns the remote socket address.
getsockopt(2) and
setsockopt(2) are used to set or get socket layer or protocol options.
ioctl(2) can be used to set or read some other options.
close(2) is used to close a socket.
shutdown(2) closes parts of a
full-duplex socket connection.
Seeking, or calling
pread(2) or
pwrite(2) with a nonzero position
is not supported on sockets.
It is possible to do nonblocking I/O on sockets by setting the
O_NONBLOCK
flag on a socket file descriptor using
fcntl(2). Then all operations
that would block will (usually) return with
EAGAIN (operation should be
retried later);
connect(2) will return
EINPROGRESS error. The
user can then wait for various events via
poll(2) or
select(2).
I/O events |
|
|
Event |
Poll flag |
Occurrence |
Read |
POLLIN |
New data arrived. |
Read |
POLLIN |
A connection setup has been completed (for connection-oriented
sockets) |
Read |
POLLHUP |
A disconnection request has been initiated by the other end. |
Read |
POLLHUP |
A connection is broken (only for connection-oriented protocols). When
the socket is written SIGPIPE is also sent. |
Write |
POLLOUT |
Socket has enough send buffer space for writing new data. |
Read/Write |
POLLIN | POLLOUT |
An outgoing connect (2) finished. |
Read/Write |
POLLERR |
An asynchronous error occurred. |
Read/Write |
POLLHUP |
The other end has shut down one direction. |
Exception |
POLLPRI |
Urgent data arrived. SIGURG is sent then. |
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An alternative to
poll(2) and
select(2) is to let the kernel
inform the application about events via a
SIGIO signal. For that the
O_ASYNC flag must be set on a socket file descriptor via
fcntl(2) and a valid signal handler for
SIGIO must be installed
via
sigaction(2). See the
Signals discussion below.
Each socket domain has its own format for socket addresses, with a
domain-specific address structure. Each of these structures begins with an
integer "family" field (typed as
sa_family_t) that indicates
the type of the address structure. This allows the various system calls (e.g.,
connect(2),
bind(2),
accept(2),
getsockname(2),
getpeername(2)), which are generic to all socket domains, to determine
the domain of a particular socket address.
To allow any type of socket address to be passed to interfaces in the sockets
API, the type
struct sockaddr is defined. The purpose of this type is
purely to allow casting of domain-specific socket address types to a
"generic" type, so as to avoid compiler warnings about type
mismatches in calls to the sockets API.
In addition, the sockets API provides the data type
struct
sockaddr_storage. This type is suitable to accommodate all supported
domain-specific socket address structures; it is large enough and is aligned
properly. (In particular, it is large enough to hold IPv6 socket addresses.)
The structure includes the following field, which can be used to identify the
type of socket address actually stored in the structure:
sa_family_t ss_family;
The
sockaddr_storage structure is useful in programs that must handle
socket addresses in a generic way (e.g., programs that must deal with both
IPv4 and IPv6 socket addresses).
The socket options listed below can be set by using
setsockopt(2) and
read with
getsockopt(2) with the socket level set to
SOL_SOCKET
for all sockets. Unless otherwise noted,
optval is a pointer to an
int.
- SO_ACCEPTCONN
- Returns a value indicating whether or not this socket has
been marked to accept connections with listen(2). The value 0
indicates that this is not a listening socket, the value 1 indicates that
this is a listening socket. This socket option is read-only.
-
SO_ATTACH_FILTER (since Linux 2.2),
SO_ATTACH_BPF (since Linux 3.19)
- Attach a classic BPF (SO_ATTACH_FILTER) or an
extended BPF (SO_ATTACH_BPF) program to the socket for use as a
filter of incoming packets. A packet will be dropped if the filter program
returns zero. If the filter program returns a nonzero value which is less
than the packet's data length, the packet will be truncated to the length
returned. If the value returned by the filter is greater than or equal to
the packet's data length, the packet is allowed to proceed
unmodified.
- The argument for SO_ATTACH_FILTER is a
sock_fprog structure, defined in
<linux/filter.h>:
-
struct sock_fprog {
unsigned short len;
struct sock_filter *filter;
};
- The argument for SO_ATTACH_BPF is a file descriptor
returned by the bpf(2) system call and must refer to a program of
type BPF_PROG_TYPE_SOCKET_FILTER.
- These options may be set multiple times for a given socket,
each time replacing the previous filter program. The classic and extended
versions may be called on the same socket, but the previous filter will
always be replaced such that a socket never has more than one filter
defined.
- Both classic and extended BPF are explained in the kernel
source file Documentation/networking/filter.txt
-
SO_ATTACH_REUSEPORT_CBPF,
SO_ATTACH_REUSEPORT_EBPF
- For use with the SO_REUSEPORT option, these options
allow the user to set a classic BPF (SO_ATTACH_REUSEPORT_CBPF) or
an extended BPF (SO_ATTACH_REUSEPORT_EBPF) program which defines
how packets are assigned to the sockets in the reuseport group (that is,
all sockets which have SO_REUSEPORT set and are using the same
local address to receive packets).
- The BPF program must return an index between 0 and N-1
representing the socket which should receive the packet (where N is the
number of sockets in the group). If the BPF program returns an invalid
index, socket selection will fall back to the plain SO_REUSEPORT
mechanism.
- Sockets are numbered in the order in which they are added
to the group (that is, the order of bind(2) calls for UDP sockets
or the order of listen(2) calls for TCP sockets). New sockets added
to a reuseport group will inherit the BPF program. When a socket is
removed from a reuseport group (via close(2)), the last socket in
the group will be moved into the closed socket's position.
- These options may be set repeatedly at any time on any
socket in the group to replace the current BPF program used by all sockets
in the group.
-
SO_ATTACH_REUSEPORT_CBPF takes the same argument
type as SO_ATTACH_FILTER and SO_ATTACH_REUSEPORT_EBPF takes
the same argument type as SO_ATTACH_BPF.
- UDP support for this feature is available since Linux 4.5;
TCP support is available since Linux 4.6.
- SO_BINDTODEVICE
- Bind this socket to a particular device like
“eth0”, as specified in the passed interface name. If the
name is an empty string or the option length is zero, the socket device
binding is removed. The passed option is a variable-length null-terminated
interface name string with the maximum size of IFNAMSIZ. If a
socket is bound to an interface, only packets received from that
particular interface are processed by the socket. Note that this works
only for some socket types, particularly AF_INET sockets. It is not
supported for packet sockets (use normal bind(2) there).
- Before Linux 3.8, this socket option could be set, but
could not retrieved with getsockopt(2). Since Linux 3.8, it is
readable. The optlen argument should contain the buffer size
available to receive the device name and is recommended to be
IFNAMSIZ bytes. The real device name length is reported back in the
optlen argument.
- SO_BROADCAST
- Set or get the broadcast flag. When enabled, datagram
sockets are allowed to send packets to a broadcast address. This option
has no effect on stream-oriented sockets.
- SO_BSDCOMPAT
- Enable BSD bug-to-bug compatibility. This is used by the
UDP protocol module in Linux 2.0 and 2.2. If enabled, ICMP errors received
for a UDP socket will not be passed to the user program. In later kernel
versions, support for this option has been phased out: Linux 2.4 silently
ignores it, and Linux 2.6 generates a kernel warning (printk()) if a
program uses this option. Linux 2.0 also enabled BSD bug-to-bug
compatibility options (random header changing, skipping of the broadcast
flag) for raw sockets with this option, but that was removed in Linux
2.2.
- SO_DEBUG
- Enable socket debugging. Allowed only for processes with
the CAP_NET_ADMIN capability or an effective user ID of 0.
-
SO_DETACH_FILTER (since Linux 2.2),
SO_DETACH_BPF (since Linux 3.19)
- These two options, which are synonyms, may be used to
remove the classic or extended BPF program attached to a socket with
either SO_ATTACH_FILTER or SO_ATTACH_BPF. The option value
is ignored.
-
SO_DOMAIN (since Linux 2.6.32)
- Retrieves the socket domain as an integer, returning a
value such as AF_INET6. See socket(2) for details. This
socket option is read-only.
- SO_ERROR
- Get and clear the pending socket error. This socket option
is read-only. Expects an integer.
- SO_DONTROUTE
- Don't send via a gateway, send only to directly connected
hosts. The same effect can be achieved by setting the MSG_DONTROUTE
flag on a socket send(2) operation. Expects an integer boolean
flag.
-
SO_INCOMING_CPU (gettable since Linux 3.19, settable
since Linux 4.4)
- Sets or gets the CPU affinity of a socket. Expects an
integer flag.
-
int cpu = 1;
setsockopt(fd, SOL_SOCKET, SO_INCOMING_CPU, &cpu,
sizeof(cpu));
- Because all of the packets for a single stream (i.e., all
packets for the same 4-tuple) arrive on the single RX queue that is
associated with a particular CPU, the typical use case is to employ one
listening process per RX queue, with the incoming flow being handled by a
listener on the same CPU that is handling the RX queue. This provides
optimal NUMA behavior and keeps CPU caches hot.
-
SO_INCOMING_NAPI_ID (gettable since Linux 4.12)
- Returns a system-level unique ID called NAPI ID that is
associated with a RX queue on which the last packet associated with that
socket is received.
- This can be used by an application to split the incoming
flows among worker threads based on the RX queue on which the packets
associated with the flows are received. It allows each worker thread to be
associated with a NIC HW receive queue and service all the connection
requests received on that RX queue. This mapping between a app thread and
a HW NIC queue streamlines the flow of data from the NIC to the
application.
- SO_KEEPALIVE
- Enable sending of keep-alive messages on
connection-oriented sockets. Expects an integer boolean flag.
- SO_LINGER
- Sets or gets the SO_LINGER option. The argument is a
linger structure.
-
struct linger {
int l_onoff; /* linger active */
int l_linger; /* how many seconds to linger for */
};
- When enabled, a close(2) or shutdown(2) will
not return until all queued messages for the socket have been successfully
sent or the linger timeout has been reached. Otherwise, the call returns
immediately and the closing is done in the background. When the socket is
closed as part of exit(2), it always lingers in the
background.
- SO_LOCK_FILTER
- When set, this option will prevent changing the filters
associated with the socket. These filters include any set using the socket
options SO_ATTACH_FILTER, SO_ATTACH_BPF,
SO_ATTACH_REUSEPORT_CBPF, and SO_ATTACH_REUSEPORT_EBPF.
- The typical use case is for a privileged process to set up
a raw socket (an operation that requires the CAP_NET_RAW
capability), apply a restrictive filter, set the SO_LOCK_FILTER
option, and then either drop its privileges or pass the socket file
descriptor to an unprivileged process via a UNIX domain socket.
- Once the SO_LOCK_FILTER option has been enabled,
attempts to change or remove the filter attached to a socket, or to
disable the SO_LOCK_FILTER option will fail with the error
EPERM.
-
SO_MARK (since Linux 2.6.25)
- Set the mark for each packet sent through this socket
(similar to the netfilter MARK target but socket-based). Changing the mark
can be used for mark-based routing without netfilter or for packet
filtering. Setting this option requires the CAP_NET_ADMIN
capability.
- SO_OOBINLINE
- If this option is enabled, out-of-band data is directly
placed into the receive data stream. Otherwise, out-of-band data is passed
only when the MSG_OOB flag is set during receiving.
- SO_PASSCRED
- Enable or disable the receiving of the
SCM_CREDENTIALS control message. For more information, see
unix(7).
- SO_PASSSEC
- Enable or disable the receiving of the SCM_SECURITY
control message. For more information, see unix(7).
-
SO_PEEK_OFF (since Linux 3.4)
- This option, which is currently supported only for
unix(7) sockets, sets the value of the "peek offset" for
the recv(2) system call when used with MSG_PEEK flag.
- When this option is set to a negative value (it is set to
-1 for all new sockets), traditional behavior is provided: recv(2)
with the MSG_PEEK flag will peek data from the front of the
queue.
- When the option is set to a value greater than or equal to
zero, then the next peek at data queued in the socket will occur at the
byte offset specified by the option value. At the same time, the
"peek offset" will be incremented by the number of bytes that
were peeked from the queue, so that a subsequent peek will return the next
data in the queue.
- If data is removed from the front of the queue via a call
to recv(2) (or similar) without the MSG_PEEK flag, the
"peek offset" will be decreased by the number of bytes removed.
In other words, receiving data without the MSG_PEEK flag will cause
the "peek offset" to be adjusted to maintain the correct
relative position in the queued data, so that a subsequent peek will
retrieve the data that would have been retrieved had the data not been
removed.
- For datagram sockets, if the "peek offset" points
to the middle of a packet, the data returned will be marked with the
MSG_TRUNC flag.
- The following example serves to illustrate the use of
SO_PEEK_OFF. Suppose a stream socket has the following queued input
data:
-
aabbccddeeff
- The following sequence of recv(2) calls would have
the effect noted in the comments:
-
int ov = 4; // Set peek offset to 4
setsockopt(fd, SOL_SOCKET, SO_PEEK_OFF, &ov, sizeof(ov));
recv(fd, buf, 2, MSG_PEEK); // Peeks "cc"; offset set to 6
recv(fd, buf, 2, MSG_PEEK); // Peeks "dd"; offset set to 8
recv(fd, buf, 2, 0); // Reads "aa"; offset set to 6
recv(fd, buf, 2, MSG_PEEK); // Peeks "ee"; offset set to 8
- SO_PEERCRED
- Return the credentials of the peer process connected to
this socket. For further details, see unix(7).
-
SO_PEERSEC (since Linux 2.6.2)
- Return the security context of the peer socket connected to
this socket. For further details, see unix(7) and
ip(7).
- SO_PRIORITY
- Set the protocol-defined priority for all packets to be
sent on this socket. Linux uses this value to order the networking queues:
packets with a higher priority may be processed first depending on the
selected device queueing discipline. Setting a priority outside the range
0 to 6 requires the CAP_NET_ADMIN capability.
-
SO_PROTOCOL (since Linux 2.6.32)
- Retrieves the socket protocol as an integer, returning a
value such as IPPROTO_SCTP. See socket(2) for details. This
socket option is read-only.
- SO_RCVBUF
- Sets or gets the maximum socket receive buffer in bytes.
The kernel doubles this value (to allow space for bookkeeping overhead)
when it is set using setsockopt(2), and this doubled value is
returned by getsockopt(2). The default value is set by the
/proc/sys/net/core/rmem_default file, and the maximum allowed value
is set by the /proc/sys/net/core/rmem_max file. The minimum
(doubled) value for this option is 256.
-
SO_RCVBUFFORCE (since Linux 2.6.14)
- Using this socket option, a privileged
(CAP_NET_ADMIN) process can perform the same task as
SO_RCVBUF, but the rmem_max limit can be overridden.
-
SO_RCVLOWAT and SO_SNDLOWAT
- Specify the minimum number of bytes in the buffer until the
socket layer will pass the data to the protocol (SO_SNDLOWAT) or
the user on receiving (SO_RCVLOWAT). These two values are
initialized to 1. SO_SNDLOWAT is not changeable on Linux
(setsockopt(2) fails with the error ENOPROTOOPT).
SO_RCVLOWAT is changeable only since Linux 2.4.
- Before Linux 2.6.28 select(2), poll(2), and
epoll(7) did not respect the SO_RCVLOWAT setting on Linux,
and indicated a socket as readable when even a single byte of data was
available. A subsequent read from the socket would then block until
SO_RCVLOWAT bytes are available. Since Linux 2.6.28,
select(2), poll(2), and epoll(7) indicate a socket as
readable only if at least SO_RCVLOWAT bytes are available.
-
SO_RCVTIMEO and SO_SNDTIMEO
- Specify the receiving or sending timeouts until reporting
an error. The argument is a struct timeval. If an input or output
function blocks for this period of time, and data has been sent or
received, the return value of that function will be the amount of data
transferred; if no data has been transferred and the timeout has been
reached, then -1 is returned with errno set to EAGAIN or
EWOULDBLOCK, or EINPROGRESS (for connect(2)) just as
if the socket was specified to be nonblocking. If the timeout is set to
zero (the default), then the operation will never timeout. Timeouts only
have effect for system calls that perform socket I/O (e.g.,
accept(2), connect(2), read(2), recvmsg(2),
send(2), sendmsg(2)); timeouts have no effect for
select(2), poll(2), epoll_wait(2), and so on.
- SO_REUSEADDR
- Indicates that the rules used in validating addresses
supplied in a bind(2) call should allow reuse of local addresses.
For AF_INET sockets this means that a socket may bind, except when
there is an active listening socket bound to the address. When the
listening socket is bound to INADDR_ANY with a specific port then
it is not possible to bind to this port for any local address. Argument is
an integer boolean flag.
-
SO_REUSEPORT (since Linux 3.9)
- Permits multiple AF_INET or AF_INET6 sockets
to be bound to an identical socket address. This option must be set on
each socket (including the first socket) prior to calling bind(2)
on the socket. To prevent port hijacking, all of the processes binding to
the same address must have the same effective UID. This option can be
employed with both TCP and UDP sockets.
- For TCP sockets, this option allows accept(2) load
distribution in a multi-threaded server to be improved by using a distinct
listener socket for each thread. This provides improved load distribution
as compared to traditional techniques such using a single
accept(2)ing thread that distributes connections, or having
multiple threads that compete to accept(2) from the same
socket.
- For UDP sockets, the use of this option can provide better
distribution of incoming datagrams to multiple processes (or threads) as
compared to the traditional technique of having multiple processes compete
to receive datagrams on the same socket.
-
SO_RXQ_OVFL (since Linux 2.6.33)
- Indicates that an unsigned 32-bit value ancillary message
(cmsg) should be attached to received skbs indicating the number of
packets dropped by the socket since its creation.
-
SO_SELECT_ERR_QUEUE (since Linux 3.10)
- When this option is set on a socket, an error condition on
a socket causes notification not only via the exceptfds set of
select(2). Similarly, poll(2) also returns a POLLPRI
whenever an POLLERR event is returned.
- Background: this option was added when waking up on an
error condition occurred only via the readfds and writefds
sets of select(2). The option was added to allow monitoring for
error conditions via the exceptfds argument without simultaneously
having to receive notifications (via readfds) for regular data that
can be read from the socket. After changes in Linux 4.16, the use of this
flag to achieve the desired notifications is no longer necessary. This
option is nevertheless retained for backwards compatibility.
- SO_SNDBUF
- Sets or gets the maximum socket send buffer in bytes. The
kernel doubles this value (to allow space for bookkeeping overhead) when
it is set using setsockopt(2), and this doubled value is returned
by getsockopt(2). The default value is set by the
/proc/sys/net/core/wmem_default file and the maximum allowed value
is set by the /proc/sys/net/core/wmem_max file. The minimum
(doubled) value for this option is 2048.
-
SO_SNDBUFFORCE (since Linux 2.6.14)
- Using this socket option, a privileged
(CAP_NET_ADMIN) process can perform the same task as
SO_SNDBUF, but the wmem_max limit can be overridden.
- SO_TIMESTAMP
- Enable or disable the receiving of the SO_TIMESTAMP
control message. The timestamp control message is sent with level
SOL_SOCKET and a cmsg_type of SCM_TIMESTAMP. The
cmsg_data field is a struct timeval indicating the reception
time of the last packet passed to the user in this call. See
cmsg(3) for details on control messages.
-
SO_TIMESTAMPNS (since Linux 2.6.22)
- Enable or disable the receiving of the
SO_TIMESTAMPNS control message. The timestamp control message is
sent with level SOL_SOCKET and a cmsg_type of
SCM_TIMESTAMPNS. The cmsg_data field is a struct
timespec indicating the reception time of the last packet passed to
the user in this call. The clock used for the timestamp is
CLOCK_REALTIME. See cmsg(3) for details on control
messages.
- A socket cannot mix SO_TIMESTAMP and
SO_TIMESTAMPNS: the two modes are mutually exclusive.
- SO_TYPE
- Gets the socket type as an integer (e.g.,
SOCK_STREAM). This socket option is read-only.
-
SO_BUSY_POLL (since Linux 3.11)
- Sets the approximate time in microseconds to busy poll on a
blocking receive when there is no data. Increasing this value requires
CAP_NET_ADMIN. The default for this option is controlled by the
/proc/sys/net/core/busy_read file.
- The value in the /proc/sys/net/core/busy_poll file
determines how long select(2) and poll(2) will busy poll
when they operate on sockets with SO_BUSY_POLL set and no events to
report are found.
- In both cases, busy polling will only be done when the
socket last received data from a network device that supports this
option.
- While busy polling may improve latency of some
applications, care must be taken when using it since this will increase
both CPU utilization and power usage.
When writing onto a connection-oriented socket that has been shut down (by the
local or the remote end)
SIGPIPE is sent to the writing process and
EPIPE is returned. The signal is not sent when the write call specified
the
MSG_NOSIGNAL flag.
When requested with the
FIOSETOWN fcntl(2) or
SIOCSPGRP
ioctl(2),
SIGIO is sent when an I/O event occurs. It is possible
to use
poll(2) or
select(2) in the signal handler to find out
which socket the event occurred on. An alternative (in Linux 2.2) is to set a
real-time signal using the
F_SETSIG fcntl(2); the handler of the
real time signal will be called with the file descriptor in the
si_fd
field of its
siginfo_t. See
fcntl(2) for more information.
Under some circumstances (e.g., multiple processes accessing a single socket),
the condition that caused the
SIGIO may have already disappeared when
the process reacts to the signal. If this happens, the process should wait
again because Linux will resend the signal later.
The core socket networking parameters can be accessed via files in the directory
/proc/sys/net/core/.
- rmem_default
- contains the default setting in bytes of the socket receive
buffer.
- rmem_max
- contains the maximum socket receive buffer size in bytes
which a user may set by using the SO_RCVBUF socket option.
- wmem_default
- contains the default setting in bytes of the socket send
buffer.
- wmem_max
- contains the maximum socket send buffer size in bytes which
a user may set by using the SO_SNDBUF socket option.
-
message_cost and message_burst
- configure the token bucket filter used to load limit
warning messages caused by external network events.
- netdev_max_backlog
- Maximum number of packets in the global input queue.
- optmem_max
- Maximum length of ancillary data and user control data like
the iovecs per socket.
These operations can be accessed using
ioctl(2):
error = ioctl(ip_socket, ioctl_type, &value_result);
- SIOCGSTAMP
- Return a struct timeval with the receive timestamp
of the last packet passed to the user. This is useful for accurate round
trip time measurements. See setitimer(2) for a description of
struct timeval. This ioctl should be used only if the socket
options SO_TIMESTAMP and SO_TIMESTAMPNS are not set on the
socket. Otherwise, it returns the timestamp of the last packet that was
received while SO_TIMESTAMP and SO_TIMESTAMPNS were not set,
or it fails if no such packet has been received, (i.e., ioctl(2)
returns -1 with errno set to ENOENT).
- SIOCSPGRP
- Set the process or process group that is to receive
SIGIO or SIGURG signals when I/O becomes possible or urgent
data is available. The argument is a pointer to a pid_t. For
further details, see the description of F_SETOWN in
fcntl(2).
- FIOASYNC
- Change the O_ASYNC flag to enable or disable
asynchronous I/O mode of the socket. Asynchronous I/O mode means that the
SIGIO signal or the signal set with F_SETSIG is raised when
a new I/O event occurs.
- Argument is an integer boolean flag. (This operation is
synonymous with the use of fcntl(2) to set the O_ASYNC
flag.)
- SIOCGPGRP
- Get the current process or process group that receives
SIGIO or SIGURG signals, or 0 when none is set.
Valid
fcntl(2) operations:
- FIOGETOWN
- The same as the SIOCGPGRP ioctl(2).
- FIOSETOWN
- The same as the SIOCSPGRP ioctl(2).
SO_BINDTODEVICE was introduced in Linux 2.0.30.
SO_PASSCRED is new
in Linux 2.2. The
/proc interfaces were introduced in Linux 2.2.
SO_RCVTIMEO and
SO_SNDTIMEO are supported since Linux 2.3.41.
Earlier, timeouts were fixed to a protocol-specific setting, and could not be
read or written.
Linux assumes that half of the send/receive buffer is used for internal kernel
structures; thus the values in the corresponding
/proc files are twice
what can be observed on the wire.
Linux will allow port reuse only with the
SO_REUSEADDR option when this
option was set both in the previous program that performed a
bind(2) to
the port and in the program that wants to reuse the port. This differs from
some implementations (e.g., FreeBSD) where only the later program needs to set
the
SO_REUSEADDR option. Typically this difference is invisible, since,
for example, a server program is designed to always set this option.
wireshark(1),
bpf(2),
connect(2),
getsockopt(2),
setsockopt(2),
socket(2),
pcap(3),
address_families(7),
capabilities(7),
ddp(7),
ip(7),
ipv6(7),
packet(7),
tcp(7),
udp(7),
unix(7),
tcpdump(8)